This invention relates to improvements in torque limiter and in an elastic pressure means for torque-transmitting steel balls of the limiter.
In general, the torque limiter functions to set a so-called critical torque, or a maximum limit of torque, more than which should not be applied between two machine elements. Whenever any excessive torque beyond the preset critical level is applied to one element on either side, the torque limiter will work to shut off the transmission of the torque to the other element. By this critical torque setting the limiter protects the machine, prevents an accident which might otherwise result in injury or death of the personnel, and provides useful means for torque control which may be incorporated in the mechanism of equipment.
The torque limiter interposed between the two machine elements must meet roughly the following three characteristic requirements:
(1) Characteristic during normal operation (with no more than the critical torque) PA1 (2) Characteristic during abnormal operation (with more than the critical torque) PA1 (3) Release characteristic
The transmission of torque between two machine elements should be such that, as though the two are solidly fastened together by a rigid body (e.g., a rigid shaft or a pair of flanges), the torque acting on one machine element is exactly transmitted to the other without any loss.
The torque limiter is a safety device as already noted, and any torque in excess of the preset critical value must be positively shut off by the limiter and kept from transmission to the other machine element. Moreover, the limiter is not desirable if its critical torque setting varies largely with the conditions under which the device is used; the limiter is desired to work accurately whenever the preset value is reached. A limiter capable of generating a shut off signal is preferred.
The characteristic of a torque limiter which has once shut off the torque transmission between two machine elements is desirably such that the two elements are completely free from each other, neither of which imparting any force to the other. The limiter is preferably of the type that needs external force for its resetting.
Heretofore, shear pins, shear plates, and shear rings have been employed as torque limiters that meet the foregoing characteristic requirements. They are of the breakdown or fracture type, so called because, when subjected to a shearing force greater than the allowable shearing stress due to application of an excessive torque, such a device is broken to shut off the torque transmission. (The transmissible torque corresponding to the allowable shearing stress is herein called the critical torque.) They have thus far been abundantly used as safety devices for many machines and appliances.
The fracture type devices are generally inexpensive and, once broken to shut off the torque transmission, they will not reset by themselves. In this sense they have been deemed more dependable for safe operation than torque limiters of other types, such as friction and ball types. The fracture type has, however, drawbacks. The most serious of the disadvantages is that, because the devices depend on the strength properties of material for the limiter action, they can sometimes be broken by the fatigue of the material itself even when the torque being transmitted is below the critical level. In certain applications, the devices do not always provide the safety in operation. In addition, they cannot in any way meet the requirements for torque control, sensing of overload, accommodation of misalignments, etc. For these reasons there has been a great need for a novel torque limiter which will replace those of the known types.
Japanese Patent Application Publication No. 23850/1963 and Patent Application Public Disclosure Nos. 31351/1973 and 51438/1974 disclose torque limiters of the so-called ball type in which balls move in axial directions to transmit or resist a torque. The proposed devices are still too unsatisfactory for torque control, overload detection, accommodation of misalignments, and other requirements.
A principal object of this invention, therefore, is to provide a novel, improved torque limiter which transmits torque and shuts it off by allowing torque-transmitting steel balls to move only radially of driving and driven members but confining the ball motion in the rotational and axial directions.
It is another object of the invention to provide an improved torque limiter which can be exactly set to a critical torque for overload release by merely turning an adjust nut.
Another object of the invention is to provide a torque limiter for which radial slots to be formed jointly by driving and driven members to receive torque-transmitting steel balls can be simultaneously made by simple machining means, with the slot-forming portions of the two members in complete register, whereby the backlash in the rotational direction of the driving and driven members is reduced to a minimum and the irregularity in rotation during torque transmission is minimized.
Still another object of the invention is to provide a torque limiter capable of accommodating the relative axial and radial displacements of the driving and driven members and the errors of deviation of the two members from the common center of rotation.
Yet another object of the invention is to provide a torque limiter having the functions of the so-called one-point setting system which can perform overload release and resetting most effectively and, once the torque transmission has been shut off, the limiter cannot reset at any other point than the shutoff point.
An additional object of the invention is to provide a torque limiter for which the overload critical torque decreases according to the rotational speed and which can simply generate an overload signal.
A further object of the invention is the provision of a torque limiter which provides high degrees of freedom for mounting, fastening, and tightening, is simple in construction, low at manufacturing cost, and has long service life.
Still a further object of the invention is to provide an elastic pressure means for torque-transmitting steel balls of a torque limiter, which can elastically and positively hold the steel balls in position and enable the limiter to accomplish the action as such smoothly and effectively.